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Abstract

Controlled lateral nonuniform doping of a great number of semiconductor devices, e.g., on a wafer, can be effected concurrently with the ion implantation step by varying the beam's angle of incidence, and by using the shadowing effects of the mask window edges.

Country

United States

Language

English (United States)

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Controlled lateral nonuniform doping of a great number of semiconductor
devices, e.g., on a wafer, can be effected concurrently with the ion implantation
step by varying the beam's angle of incidence, and by using the shadowing
effects of the mask window edges.

Lateral nonuniform doping profiles of this kind are used to increase the
device performance of the semiconductor elements. For example, in unipolar
and lateral bipolar transistor structures, a doping gradient from source-to-drain
and emitter-to-collector (drift transistor), respectively, is frequently required. The
method proposed also permits doping only a partial area, e.g., the gate zone of a
field-effect transistor (FET) in the direction of the channel length or the channel
width.

A lateral NPN transistor is shown schematically in the figure. Wafer 1 is
covered by an oxide mask 2 having a thickness H (approx. 10 000 Angstroms),
but any other mask material such as photoresist would be equally suitable.
Windows (having a width W) are opened for base implantation, using B+ ions
with an energy of < 150 keV for NPN structures or, e.g., P+ ions for PNP
transistors. If the ion beam impinges on the surface at an angle of Phi(< 90
degrees), the shadowing of window edge 3 prevents the implantation of the base
area between B and C. Only the area between A and B is doped by ion
implantation.